The first step would be to consult a good reference to determine which rhododendrons, species and/or hybrids, at least have the potential to reach the height you seek and are adaptable to your particular climate. That being said, rhododendrons in general do not grow rapidly and attempting to push them with extra fertilizer, etc. usually results in weak growth which is vulnerable to winter damage. The native species maximum and catawbiense can grow to great heights but this takes time and patience.

Also, you should consider that anything that grows fast to start with is going to keep growing fast. You'll have your canopy in a couple of years, but then you'll have a forest.

I agree too that the best place to start with something like that is a book. Once you pick the plants you want, (as opposed to what people on the forum think you should want!) find suppliers for them through the magic of Google.

I have a large Margerite Mack growing in a raised bed next to a high bank. It is shaded from all but dappled afternoon sun by redwoods and tanoaks. It's about 15 years old and a splendid bloomer, except when it had thrips which took me about 3 years to finally figure out and get rid of. It's growing in the native soil which is sandy and acidic from the redwoods. I do fertilize. I water it once a week and I can see the water drain from the bottom of the rock wall holding the bed. I noticed at the end of this summer that it had some branches with collapsed, dying leaves. I took them to two nurseries. Both showed me how dry the leaves were and diagnosed root rot. Neither could tell me how that could happen in a raised bed. A few days after this it rained and seemed to me the rest of the plant perked up. So a few days later, instead of using the stream of water from a hose as I usually do, I attached a little sprinkler head and watered that way, thinking maybe the water is flowing right through the bed and out the bottom. What else should I try? I'd hate to loose this rhododendron.

I think you are on the right track. Your bed is well drained. However, it is so well drained that it is not soaking up the water you are applying weekly. Since you apparently live on the west coast in a Mediterranean climate, the sprinkler watering each week would work best. The symptoms are similar for drought and root rot. With drought, the die back is more isolated to specific branches. With root rot it kills the entire root system and hence the entire plant.

nedrock, why don't you consider one of the oozing type inexpensive drip hoses (WalMart, HomeDepot, etc), or drip irrigation of some kind. A slow soak is going to benefit your rhododendron more than you standing there with the hose in your hand.

I have several of the soaking hoses wound around in multiple beds, some for as long as 12 years now...I don't pick them up for winter in my climate. I turn them on once every 7 - 10 days (when we haven't had rain) for several hours for my clay based soil. You can experiment with your own soil (sandy is going to soak in deeper more quickly, and dry out again faster) by letting the hose drip/ooze, checking depth of moisture with a trowel or shovel...it could take anywhere from 2 to 12 hours for a thorough soak - with very little water waste.